Opening 3DM files can be a bit tricky since they are not a commonly supported file format. 3DM files are primarily associated with Rhinoceros 3D, a popular computer-aided design (CAD) software. Here are some methods and tools you can use to open 3DM files:

1. Use Rhinoceros 3D Software:

If you have access to Rhinoceros 3D software, you can simply open 3DM files by launching the program and using the 'Open' feature to browse and select the 3DM file you want to access.

2. Use a File Viewer/Editor:

There are various third-party file viewers and editors available that support 3DM files. One such option is the 3D-Tool Free Viewer which allows you to open, view, and analyze 3D models and 3DM files without the need for Rhinoceros 3D.

3. Convert 3DM to a More Common Format:

If you are unable to access a suitable program to open 3DM files, you can consider converting the file to a more commonly supported format such as STL, DWG, or OBJ. There are online file conversion services and software tools available that can assist with this process.

4. Use CAD Software:

Some other CAD software applications may also have limited support for opening and viewing 3DM files. Explore options such as Autodesk Fusion 360, SketchUp, or FreeCAD to see if they can provide a solution for accessing 3DM files.

5. Seek Help from a Professional:

If you are working on an important project and struggling to access a 3DM file, consider seeking assistance from a professional with experience in CAD software. They may be able to provide guidance or assistance in opening and working with the file.

Opening 3DM files can be challenging due to their specific association with Rhinoceros 3D, but with the right tools and methods, you can successfully access and work with these files. Whether you have access to the original software, use a third-party viewer, or convert the file to a different format, there are options available to meet your needs.